Another way to check the receiver battery, aid if you only have a digital voltmeter (not ESV.. just a standard digital voltmeter) then you can plug a servo extension into a vacant slot on the RX, attach the voltmeter leads to the wires of the servo extension (black and red wires) and turn on the TX then the RX.

The load is there because this is what the battery "sees" on a typical flight. Wiggle the TX sticks around a bit and you will see the voltage vary as you adjust the load by moving servos. Then, follow the guidelines for what voltage is safe to fly with.